This invention relates, to a tethered buoyant support for risers to a floating production vessel, the tethered buoyant support being at a mid-water location for supporting the riser pipe catenaries.
A lower J-shaped catenary extends from the seabed to the support, and an upper U-shaped catenary extends from the support to the vessel floating at the surface. The riser system with a single buoyant support can comprise multiple riser pipes, all of them with lower and upper catenaries. Previous similar catenary riser systems have been described in EP 251488 and UK 2295408.
In all water depths, the upper catenary is usually fabricated from flexible pipe or xe2x80x98flexpipexe2x80x99. Flexpipe is able to absorb vessel motion in waves without being vulnerable to fatigue failure, and has been used for most risers to floating production vessels in service in 1998. Flexpipe is here defined as high pressure flexible pipe, which usually includes helical high-strength windings (such as steel or possibly carbon fibre) to re-inforce polymer tubes or an elastomer matrix.
In deep water (greater than 500 m) it is desirable to fabricate the lower catenary from steel pipe rather than flexpipe, due to the steel pipe having long length relative to its diameter (the length being around 1000 times greater than the diameter, or more). Steel catenary riser (SCR) technology to a tension leg platform (TLP) is described in a technical paper entitled xe2x80x98Design and Installation of Auger Steel Catenary Risersxe2x80x99 presented at the Offshore Technology Conference in Houston, May 1994, paper number OTC 7620. UK 2295408 describes the application of SCRs with a tethered buoyant mid-water support, rather than to a TLP.
Installation of tethered buoyant supports in 130 m water depth offshore North West Australia is described in xe2x80x98Installation of the Griffin FPSO and Associated Subsea Constructionxe2x80x99, paper presented at the Floating Production Systems Conference, in London, Dec. 8-9, 1994. Each cylindrical buoy was 3.7 m diameter and up to 14 m long with chain tethers from each end down to a seabed base. The buoy was positioned approximately 45 m below the sea surface. The buoys carried arches for supporting flexpipe risers and umbilicals, and the arch radius was approximately 3 m, with the buoy cylinder positioned centrally under the arches (at least before installing flexpipe risers).
In deep water, the tension at the top of the lower J-shaped catenary extending from the mid-water support to the seabed can be very large due to the submerged weight of the long length of the lower catenary pipe. The paper OSEA-94113, xe2x80x98A Hybrid Riser for Deep Waterxe2x80x99 presented at the Offshore South East Asia Conference, Singapore, Dec. 6-9, 1994, suggests that multiple SCRs from a mid-water support located 100 to 150 m below surface in 1200 m depth, will have a combined submerged weight of 1200 tonnes. The paper OTC 8441xe2x80x94xe2x80x98Integrated Asymmetric Mooring and Hybrid Riser System for Turret Moored Vessels in Deep Waterxe2x80x99, presented at the Offshore Technology Conference, Houston, May 5-8, 1997xe2x80x94describes a tethered riser buoy in 1000 m water depth for supporting up to approximately 800 tonnes of load from 15 risers and umbilicals. Paper OTC 8441 suggests that a concrete buoy for this application should be 8 m diameter and 80 m long, and should generate 1200 tonnes of tether tension to provide adequate lateral stability.
The problem with hanging a load of 800 to 1200 tonnes from a circular section buoy with a centrally-positioned support arch of 3 to 4 m radius is that the moment of up to 4800 tonne-meters will tend to rotate the buoy. Also, the rotation could bend the upper ends of the risers unless they are hanging from a xe2x80x98hingedxe2x80x99 (i.e. free) support.
Even if the lower riser portion submerged weight can be reduced by adding a low density coating, or by using pipe-in-pipe construction with a gas-filled annulus, the hanging weight is still likely to be hundreds of tonnes.
The invention has therefore been made with these points in mind.
According to the invention there is provided a mid-water tethered buoyant support assembly for a riser system for use in water to bring fluids from seabed equipment to a production vessel at the surface, the tethered buoyant support assembly comprising at least two tethers from seabed anchors, at least one beam assembly extending between and connected to the tops of the tethers, buoyancy means to maintain tension in the tethers, and hangers for lower riser portions mounted at spaced positions along the beam assembly, each hanger being positioned so that the line of action of the tension due to the weight of the suspended lower riser portion is close to or on a line extending between the connections of the beam to the tethers, to minimise or eliminate turning moment to the beam assembly tending to cause rotation of the beam around its major axis as a result of the weight of the suspended lower riser portion.
Such an assembly supports the lower riser weight with minimum tendency to cause rotation of the tethered buoyant support. In addition it is possible to provide a large amount of adjustable buoyancy at the support form which is readily fabricated. Further, there is resistance to rotation of the support when flexpipe upper catenaries are added.
Advantageously, the distance between the line of action of the tension of a lower riser portion and the line extending between the tops of the tethers is at most one quarter of the distance from the centre of buoyancy of the buoyancy means to the tops of the tethers. More advantageously, the distance between the line of action of the tension of any lower riser portion and the line extending between the tops of the tethers is at most one twentieth of the distance from the centre of buoyancy of the buoyancy means to the tops of the tethers.
The tethered buoyant support may include joining and/or guiding and/or aligning means for upper riser portions mounted on the beam structure at spaced positions corresponding to the hangers.
The vertical tethers can be similar to the tubular tethers used for TLPs, which are generally steel tubes and have elastomeric bearings at the connection to the seabed anchors. Similarly, the connections of the tethers to the beam can be elastomeric bearings.
The horizontal beam structure can be two tubes around 2 m diameter and spaced around 4 m apart by minor tubular members in the manner of a braced truss around 50 m in length, and the hangers can be similar to those described in European patent EP 0,251,488 or UK patent application 2,323,876. The means for joining or guiding or aligning the upper riser portions to their corresponding lower riser portions can comprise arches for supporting flexible pipe, or inverted U-shaped piping spools, or funnels or guide posts for aligning connectors.
The main buoyancy tanks can be circular cylinder-shaped with the major axis vertical or rectangular block-shaped, and with the attachment to the beam at the centre of the lower face. The tanks may have dimensions around 20 m highxc3x9710 m diameter (1570 cu.m. displacement) if this large amount of buoyancy is needed, depending on the total riser weight to be supported. The inside of the tanks can be partitioned to allow progressive increase of the buoyancy by de-ballasting pairs of partitions to maintain the buoy and beam close to vertical. Each de-ballastable compartment has suitable valves to allow injection of air or nitrogen to the top, and ejection of contained water at the bottom, with minimum overpressure of the gas above external water pressure.